A known inductively-driven electrodeless high-intensity discharge (HID) lamp comprises an arc tube having a wall of light-transmissive material. An excitation coil surrounds the arc tube and is energizable with radio frequency current to develop a toroidal arc discharge within the arc tube. When such a lamp is relied upon as the light source for a luminaire, the lamp may be supported within an enclosure that includes a wall portion surrounding the lamp and terminating in a opening through which light from the lamp is transmitted. Aligned with this opening, there may be a refractor of light-transmissive material for receiving the light passing through the opening and typically having prismatic surfaces especially shaped to distribute this light in a desired pattern.
A large portion of the light transmitted through the refractor is reflected light and, more specifically, light developed by the toroidal arc discharge and reflected from reflecting means provided within the luminaire. In most prior luminaires the principal reflecting means is constituted by one or more surfaces of the above-described enclosure which have good light-reflective characteristics. Such surfaces of the enclosure are configured so that light rays from the lamp which strike these surfaces are reflected from the surface through the refractor at the forward end of the enclosure. In the type of luminaire that we are concerned with, i.e., one that uses an electrodeless discharge lamp as its light source, there is a significant problem if the principal reflecting means is of the above-described type, i.e., reflecting surfaces on the enclosure. More specifically, in the case of the inductively-driven, electrodeless HID lamp, the presence of the excitation coil that surrounds the arc tube constitutes an impediment to the passage of light rays from the toroidal arc to the principal reflecting surfaces and also an impediment to the passage of light rays from the principal reflecting surfaces through the refractor at the forward end of the enclosure. Such blockage can significantly reduce the efficiency of the luminaire.
While it is possible to design the principal reflecting surfaces so that light reflected therefrom will follow paths that avoid the excitation coil and other associated impediments, this approach typically requires that some of the light rays be reflected more than once off these surfaces before exiting through the forward end of the enclosure. This is disadvantageous because each reflection involves some loss of light, typically about 10%, which reduces the efficiency of the luminaire. Secondly, the multiple reflection approach is disadvantageous because its use results in light rays arriving at individual points on the refractor at widely varying incident angles, and this tends to reduce the effectiveness of the refractor in functioning as desired to direct light via predetermined paths as it emerges from the refractor. This problem is further discussed in the next paragraph.
Another disadvantage of relying upon principal reflecting surfaces on or near the enclosure is that these reflecting surfaces must be of a specular character in order to effectively cooperate with the optics of the refractor. More specifically, such cooperation is best assured if substantially all the light striking a given point on the prismatic surface of the refractor approaches this point via a precise, predetermined path. This is possible if substantially all the reflected light reaching the prismatic surface is reflected light from carefully designed specular reflecting surfaces. But if the reflecting surfaces are far from the light source and especially if they are diffuse reflecting surfaces, the light arriving at the refractor from the reflecting surfaces will approach each point on the prismatic surface of the refractor via many diverse paths. This significantly detracts from the desired ability of the prismatic surface to direct this light via a precise path as it emerges from the prism. Accordingly, diffuse reflective surfaces are avoided in the typical refractor-containing luminaire.
An example of a light source utilizing an electrodeless discharge arrangement and reflective surfaces in close proximity to the light source can be found in U.S. Pat. No. 3,248,548 issued to Booth et al on Apr. 26, 1966. It can be seen from this patent that a laser generating device provides a reflective coating over substantially the entire surface of the arc tube having only an aperture opening through which the light is output thereby affecting a laser delivery.
Therefore, it would be advantageous to provide, in a luminaire that includes as its light source an electrodeless discharge lamp, reflecting means so constructed that light rays from the arc discharge within the lamp can reach the reflecting means and be reflected therefrom through the forward end of the luminaire enclosure without significant interference from the excitation means (e.g., the excitation coil) of the lamp and without requiring multiple reflections in order to avoid the excitation means when passing from the reflecting means to the forward end.
In U.S. Pat. Nos. 3,763,392--Hollister and 3,860,854--Hollister, there is disclosed an inductively-driven, electrodeless HID lamp in which a reflecting chamber is provided about the arc tube and between the arc tube and the exciting coil. The outer wall of this reflecting chamber is of a reflective material or is coated to be reflective and thus acts as a reflector for light generated within the arc tube. A disadvantage of this construction is that the reflector is still spaced a substantial distance from the arc tube and thus is unable to cooperate as effectively as might be desired with the optics of any refractor in view of the above-described tendency of distant reflecting surfaces to cause the reflected light to approach each point on the refractor via many diverse paths.
In U.S. Pat. No. 4,910,439--El-Hamamsy et al, there is disclosed an electrodeless discharge lamp comprising an arc tube and a reflecting chamber (40) positioned in a location similar to that described above for the reflecting chamber of the Hollister patents. Within this chamber 40 of El-Hamamsy are mounted discrete reflecting elements 44 and 44' spaced from the arc tube and acting as principal reflectors for light generated within the arc tube. These reflectors, like those of Hollister, are still spaced a substantial distance from the arc tube and thus are subject to substantially the same disadvantages as pointed out above in connection with the reflectors of Hollister.
As pointed out in more detail hereinafter, the light reflected off this reflective coating on the arc tube wall can be redirected after such reflection, and such redirection can be accomplished by redirecting means in the form of either a secondary reflector or a refractor. In either case, an object of our invention is to cause light from the source that is reflected off the arc-tube reflective coating to approach individual points on the surface of the redirecting means at approximately the same incident angle as the direct light from the source approaches that point.